1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer and a copier.
2. Description of Related Art
An image forming apparatus such as an electrophotographic printer and a copier forms an image on a recording medium by performing an image forming cycle including a charging process in which a photosensitive drum is imparted with electric charge, an exposing process in which an electrostatic latent image is formed on surface of the photosensitive drum in accordance with image data, a developing process in which the electrostatic latent image is visualized by transferring toner from a developing roller on which a thin toner layer is formed to the electrostatic latent image, a transferring process in which the visualized image (toner image) is transferred to the recording medium such as paper, and a fixation process in which the toner image transferred to the recording medium is fixed by applying heat and pressure to the toner image.
The amount of toner on the developing roller is environment-dependent, that is, it varies with surrounding temperature and humidity. The variation of the amount of toner on the developing roller causes variation of density of the toner image formed on the recording medium. Generally, the amount of toner on the developing roller of the image forming apparatus when operating under low temperature and low humidity, for example under temperature of 10 degrees C. and humidity of 20%, is greater than that when operating under normal temperature and normal temperature. If the amount of toner on the developing roller becomes too great, density non-uniformity or fogging occurs and image quality is degraded.
Accordingly, as disclosed, for example in Japanese Patent Application Laid-Open No. 11-7198, it is known to control the voltage to be applied to a toner supply roller for supplying toner to the developing roller in order that the amount of toner on the developing roller is kept unchanged despite temperature change or humidity change. In this case, by stabilizing the amount of electric charge of toner to be supplied to the photosensitive drum, fogging and density non-uniformity can be even further reduced.
However, experience shows that density unevenness or thin spots may occur in a printed image even if the voltage applied to the toner supplying roller is controlled depending on the surrounding temperature and humidity after such an image forming apparatus executes considerable number of image forming cycles on a continuous basis.